The present invention relates generally to pistons for internal combustion engines for motor vehicles, etc. and more particularly, to piston ring grooves thereof.
In recent years, pistons for internal combustion engines for motor vehicles are made of aluminum alloy instead of cast iron so as to achieve a weight reduction in view of requirements of high power and high performance. Grooves for receiving piston rings are formed on an outer peripheral surface of the piston which faces an inner wall of a cylinder bore. A top ring groove, the nearest one of the piston ring grooves with respect to a combustion chamber, suffers great wear by a piston ring (top ring) due to exposure to a high temperature and direct receiving of a combustion pressure in particular. Thus, aluminum micro-welding is apt to occur between the top ring groove and tile top ring.
Various techniques have been proposed to prevent such aluminum micro-welding: 1) Reinforcement of the surface of the top ring groove by compounding inorganic fiber aggregate (see JP-A 59-201953); 2) Application of hybrid MMC (metal matrix composites) by In-Situ process to the pistons (see Automotive Technique No. 891,056 published in May, 1989 by Automotive Technique Society); 3) Reinforcement of the surface of the top ring groove by compounding nickel porous material (see JP-B2 3-30708); 4) Reinforcement of tile surface of the top ring groove by an alumite-treated layer (see JP-A 1-190951); 5) Formation of a copper-alloy layer by electron beam fusion treatment on the surface of the piston ring groove (see No. 1 Technical Revue published in 1988 by Mitsubishi Motor Co., Ltd.; 6) Formation of a ring support portion in the top ring groove by casting aluminum alloy around Ni-resist cast iron as alfin-treated.
However, the above prior arts present the following inconveniences: The prior arts 1)-3) need the use of a high-pressure solidification method in view of applied materials such as inorganic fiber, etc. with respect to a forming method. This results in a rise of manufacturing cost and a restriction of the piston shape.
The prior art 4) contributes to improvement of a micro-welding resistance with the piston ring due to presence of the alumite-treated layer, but fails to provide a sufficient wear resistance. On the contrary, the prior art 5) may fall to provide a sufficient micro-welding resistance.
The prior art 6), a technique being applied from long ago, ensures a wear resistance and a micro-welding resistance, but cannot avoid a weight increase due to cast iron making.
It is, therefore, an object of the present invention to provide pistons for internal combustion engines which provide sufficient wear resistance and micro-welding resistance without any increase in weight and manufacturing cost.